Package materials like styrofoam* or polystyrene foam* can be used for filling up
bigger models and for creating stocky armatures.

* When baking styrofoam, it may give off toxic fumes!
I haven't had this with the syrofoam I use, but I've been told that most styrofoam
in the US give off toxic fumes when heated. In Europe they have a different kind of styrofoam.
If you are unsure if your type of package material is safe, don't use it! Use tinfoil instead.